By Bonnie Ennis, Colorado State University Cooperative
Extension agent, horticulture

You and I undoubtedly have turned up the heat more than once this
winter to survive the cold. Or we've bundled up in coats, hats, mittens and heavy socks to
stay warm.

Insects also devise ways to stay warm in winter. Some go dormant, into
a state called "diapause," while others remain active. Insects that spend the
winter in diapause withstand a greater range of low temperatures than those that remain
active -- some to temperatures as low as -94o F, which is about the lowest
reported temperature for natural environments.

With few exceptions, insects that go dormant for the winter fit into
two classes: freeze-susceptible and freeze tolerant.

A freeze-susceptible insect avoids freezing altogether by depending
heavily on antifreeze compounds, called cryoprotectants, to supercool body fluids and
tissues above their freezing point. Ethylene glycol, the same compound found in antifreeze
for cars, is the most common cryoprotectant.

Freeze-tolerant insects do not really freeze, at least not totally.
Just the fluid, which bathes their living cells, freezes. This freezing process forces
water out of the living cells thus lowering the freezing point even further. (Smaller
amounts of water freeze at lower temperatures than larger amounts of water.)

Freeze-tolerant insects aren't necessarily more cold tolerant than
freeze-susceptible insects. Each class of insects just has its own unique survival
strategy.

Both types of insects also are affected by other conditions that affect
their rate of winter survival: size, moisture, nutrition, temperature, stage of growth and
species.

Size

Insects essentially are little bags of water. Their ability to
supercool depends, in part, on how much water they contain. The smaller the amount of
water, the greater their ability to supercool without freezing. Smaller arthropods (ants,
insect eggs, tiny spider mites) can supercool to colder temperatures than larger species,
such as grasshoppers. This phenomenon also is true between individuals within a species. A
smaller housefly may be more cold hardy than a larger housefly.

Moisture

Partial drying can be beneficial for some insects. The freezing point
of a housefly is lowered by 50o F after dehydration. For other insects, such as
boxelder bugs, dehydration has no cold weather benefits.

Nutrition

An insect with an empty gut will survive lower subfreezing temperatures
than an insect with a full gut. This is because food in the gut attracts water that can
freeze and promote growth of ice crystals.

Temperature

The intensity and duration of cold and freezing, as well as exposure
time to cold is critical to insect survival. If exposed long enough, an insect will die at
moderately cold temperature.

Stage of Growth

Insects pass through several stages of growth before beginning a new
generation. For the most part, each insect species overwinters in a stage of growth best
adapted to cold temperatures.

For example, non-feeding growth stages (eggs, pupae) survive colder
temperatures than feeding growth stages (larvae and adults, although not all larvae and
adult stages feed actively.) Some insects improve chances of winter survival by building
protective structures, such as cocoons or pupae, at a specific growth stage. This reduces
surface moisture contact that could cause freezing.

Species and Migration

Insects are adapted to different climates. Whiteflies evolved in
tropical and subtropical climates and perish in Colorado unless they can overwinter in a
heated building. Tomato hornworms spend the Colorado winter in the pupal stage (inside a
hardened case) and buried several inches deep in the soil.

Other insect species migrate to warmer climates for the winter. The
Monarch and Painted Lady butterflies migrate south in late summer, then return in spring
with warmer weather.

Most species, however, move only to a different and generally nearby
habitat, such as from fields to woodlands, and in agricultural areas from fields to
groves, hedges or shelterbelts. Other insects overwinter in their summer sites.

Environment

Winter conditions and suitably protected habitats can be critical to an
insect's winter survival. Snow cover offers good insulation from winter cold. Light,
fluffy snow provides more insulation than packed snow. The north side of a tree maintains
the same temperature as the air, but the south side of a tree may be several degrees
warmer on a sunny day. Wet substrates (soil, leaves) cool slower than dry substrates;
saturated substrates may suffocate an overwintering insect.

With so many variables affecting winter insect survival, it is
difficult to predict such rates from one year to the next. Just the same, we can use
winter conditions to monitor the development of certain agricultural pests.

Mild conditions, for example, will produce an earlier spring migration
of green peach aphids (you may see this little pest on your lettuce and spinach leaves in
summer). Long periods of sub-zero temperatures result in winter mortality of some
grasshoppers. By understanding these factors, we can more accurately time pest control
procedures for some insects.